The present invention relates to a gas sensor for measurement of oxygen and/or the air-to-fuel lambda ratio and hydrocarbons and/or carbon monoxide in gas mixtures. The gas sensor has a reference electrode representing a constant oxygen partial pressure, an oxygen ion-conducting solid electrolyte, and at least two measuring electrodes, the measuring electrodes and the reference electrode being mounted directly on the solid electrolyte and having electrical leads for connection and for take-away of the electrical measurement signals. The invention also relates to applications for the gas sensor and to a measurement method.
A gas sensor of a this type is known, e.g., from German published patent application DE 195 34 918 A1. The sensor therein has two electrodes constructed as mutually engaging comb structures (see FIG. 1), which are arranged on the side of the solid electrolyte facing the gas being measured (hereinafter xe2x80x9cmeasurement gasxe2x80x9d), and a reference electrode is provided opposite thereto on the reference air side. That invention is directed mainly to a reliable seal to ensure that no effects are exerted on the operation and performance of the two electrodes (sensor contacts) provided on the measurement gas side of the solid electrolyte. This construction makes possible a voltammetric measurement of two gas components in a gas mixture.
In addition, a gas sensor of the generic type is known from German published patent application DE 36 10 366 A1, in which a plurality of electrochemical measuring cells are arranged on a tubular support. This device allows only gaseous pollutants to be measured (not oxygen). The evaluation of the measurement signals takes place based on the characteristics of the pollutant concentrations.
Furthermore, a gas sensor of this type is known from German Patent DE 41 09 516 C2. In this device, the solid electrolyte is constructed in the shape of a platelet, on one side of which an electrode is applied which functions as a reference electrode, and on the opposite side of which at least two measuring electrodes are applied, which interact with various components of a gas mixture. The platelet-shaped sensor is built into a housing, which is then to be installed as a gas probe in the exhaust gas duct of a motor vehicle, more specifically perpendicular to the flow direction of the exhaust gas.
This probe functions without a reference gas, which is required for obtaining an electrode potential independent of the environment. However, such electrodes are not stable with respect to their electrochemical potential, especially when the mixture composition changes from lean to rich. In addition, with configurations of the sensor design which are not rotationally symmetric, it is very hard to realize a stable and uniform temperature distribution over the entire surface. A similar, relatively complicated sensor is also known from German published patent application DE4243 734 A1.
In view of the above, an object of the present invention is to provide a gas sensor with which at least two gaseous components can be reliably detected simultaneously over a wide range of gas mixtures, and which also ensures a stable reference signal with the aid of ambient air, which additionally and, if necessary, allows the influence of the oxygen concentration by adding or removing oxygen at the respective measuring electrodes.
These objectives are achieved according to the present invention, wherein the solid electrolyte is constructed with one side exposed to the measurement gas and with a reference gas side separated from the measurement gas. The arrangement of the electrodes with the reference electrode on the reference gas side and with the at least two measuring electrodes on the measuring gas side is constructed such that one of the reference electrodes is assigned to at least one measuring electrode, which forms the anode of this electrode pair. The electrode pair is adapted to apply a voltage or a current for pumping of oxygen, and the arrangement simultaneously transmits at least two measurement signals, which correspond to different gaseous components. Alternatively, at least one of the reference electrodes can be assigned to at least two measuring electrodes, which are arranged spaced from one another on the same solid electrolyte.
According to the measurement method of the invention, oxygen is pumped from the reference gas side to the measurement gas side, whereby an oxygen excess is formed on the measurement gas side, and a difference signal is measured between two different measuring electrodes. Other advantageous amplifications of the invention, as well as the use of the gas sensor of invention, are described below and set forth in the dependent claims.
Advantageously the solid electrolyte, generally provided as a solid electrolyte body of virtually any desired shape, is constructed as a small tube closed at one end, which has on its inner wall a reference electrode, positioned as close as possible to the closed end, and a plurality of electrodes arranged on the outer side, exposed to the measurement gas. The solid electrolyte consists, e.g., of partially or fully stabilized ZrO2  or of CeO2. The arrangement of at least two independent measuring electrodes on the solid electrolyte guarantees the simultaneous detection of at least two measurement signals which correspond to at least two different gas components. Since a tubular solid electrolyte with a circular cross-section is used, the disturbances at an installation point perpendicular to the exhaust gas flow are thereby minimized, so that the measurement gas flows around the sensor in a relatively uniform manner. Accordingly, the gas components being measured arrive at the measuring electrodes practically without a delay, and the disturbing turbulence is avoided.
If the gas sensor is used at temperatures below 400xc2x0 C., it is advantageous to provide the sensor with a heating element. The heating element for this purpose can be applied as a heating conductor, likewise on the outer side of the solid electrolyte, wherein, however, in order to avoid a short circuit, an electrically insulating layer is arranged between the heating conductor and the solid electrolyte.
Expediently, at least one of the electrodes used as a measuring electrode on the outer side of the solid electrolyte tube, closed at one end, is made of a catalytically active material, wherein different measuring electrodes can have different catalytically active materials. Consequently, the at least one measuring electrode is particularly suited for the potentiometric oxygen measurement according to the principle of a Nernst probe. In contrast, the second measuring electrode is made of a catalytically inactive material. This electrode is preferably used for detecting hydrocarbons.
It is advantageous if the surface of the measuring electrodes facing the measurement gas is covered with a preferably porous diffusion layer which, for example, can be made of aluminum oxide, spinel, or magnesium oxide, and which can have a different layer thickness over each measuring electrode, in order to be able to influence the oxygen residence time aimed at.
The reference electrodes assigned to the mutually spaced apart measuring electrodes can be divided into mutually spaced apart partial reference electrodes.
By using different catalytically active electrode materials for the adjacent measuring electrodes, assigned to the same reference electrodes, a gas-symmetrical differential measurement can be conducted between two measuring electrodes, wherein the selectivity with respect to hydrocarbons can be improved, for example by the choice of the oxygen pressure at these electrodes. At the same time, cross-influences arising through changing lambda are avoided. By the connection of a measuring electrode as the anode with respect to the reference electrode assigned to it, the targeted amount of oxygen can be pumped from the reference gas side to the measurement gas side by the application of a voltage or a current.
As catalytically active materials platinum or platinum alloys have proven satisfactory. Furthermore, rhodium or palladium are also suitable as catalytically active electrode materials. For the catalytically inactive materials, which should be used for the second measuring electrode, gold and gold alloys, as well as metal oxides have proven satisfactory. The catalytically inactive metal oxides are exemplified by mixed-conductivity perovskite compounds of the general formula Ln1xe2x88x92zA1xe2x88x92xBxO3, wherein Ln is a lanthanide cation, A is an element selected from the group Mn, Cr, Co, Fe, Ti, or Ni (preferably Cr or Ti), and B is an element selected from the group Ga, Al, Sc, Mg, or Ca.
By varying the composition of the measuring electrodes, it is possible to bring various gas components to the electrodes for interaction. The gas sensor according to the invention is therefore particularly suitable for applications for simultaneous measurement in gas mixtures where oxygen or lambda and hydrocarbons or carbon monoxide are to be found.
The measurement of oxygen is preferably carried out at the catalytically active measuring electrode, wherein the potential which arises, dependent on the concentration of oxygen in the measurement gas, is measured against the air reference electrode. This measuring electrode sets the measurement gas into equilibrium, and the voltage detected at this electrode gives a signal corresponding to that of known lambda probes.
At the measuring electrodes, which are preferably made of a catalytically inactive material, such as mixed-conductivity metal oxides (mixed-oxides), a voltage can likewise be detected against the (air) reference, which is determined from the concentration of unburned components, i.e., from the concentration of hydrocarbons or carbon monoxide in the exhaust gas.
In the method according to the invention, oxygen is pumped from the reference gas side to the side is exposed to the measurement, gas and a difference signal is measured between two different measuring electrodes.